Roles of Free Electrons and HO in the Optical Breakdown-Induced Photochemical Reduction of Aqueous [AuCl].

Free electrons and HO formed in an optical breakdown plasma are found to directly control the kinetics of [AuCl] reduction to form Au nanoparticles (AuNPs) during femtosecond laser-assisted synthesis of AuNPs. The formation rates of both free electrons and HO strongly depend on the energy and duration of the 800 nm laser pulses over the ranges of 10-2400 μJ and 30-1500 fs. By monitoring the conversion of [AuCl] to AuNPs using in situ UV-vis spectroscopy during laser irradiation, the first- and second-order rate constants in the autocatalytic rate law, k and k, were extracted and compared to the computed free electron densities and experimentally measured HO formation rates. For laser pulse energies of 600 μJ and lower at all pulse durations, the first-order rate constant, k, was found to be directly proportional to the theoretically calculated plasma volume, in which the electron density exceeds the threshold value of 1.8 × 10 cm. The second-order rate constant, k, was found to correlate with the measured HO formation rate at all pulse energies and durations, resulting in the empirical relationship k ≈ HO. We have demonstrated that the relative composition of free electrons and HO in the optical breakdown plasma may be controlled by changing the pulse energy and duration, which may make it possible to tune the size and dispersity of AuNPs and other metal nanoparticle products synthesized with femtosecond laser-based methods.